Plaster interior walls

Summary: Learn how to plaster interior masonry walls, apply the basecoat, score the basecoat and plaster an external corner.

Plastering is the traditional way of covering walls but it does require practice to master the techniques. Additionally, you do need to be reasonably fit, as it is quite strenuous work. Firstly, assess the job to decide whether it would be more cost effective to engage the services of a professional plasterer. As an alternative to plastering you may want to consider dry lining the walls.

Before you start

There are several different types of plaster available. Professionals use a gypsum-based plaster that needs to be mixed correctly and dries quickly. If a novice, you would be better advised using a ready-mixed plaster.

Whichever plaster you decide to use, remember to wear goggles and a respiratory mask when mixing, as the dust can be harmful. Be careful not to contaminate the mixture with small pieces of debris during mixing as these make it difficult to apply to the walls and get a smooth finish. Do not mix too much plaster at once, especially if it is a hot day, as it will dry out too quickly.

You can plaster directly onto plasterboard stud walls or over a basecoat or undercoat that has been applied to masonry walls. This project is looking at plastering an interior masonry wall with a base coat and top coat.

Basecoats

For the smoothest finish on a masonry wall, you should apply a basecoat before the finishing plaster. There are various basecoats available to choose from, but all are applied in the same way. The easiest basecoat to apply is Undercoat Plaster, which is also the best insulator.

However, if you have damp walls, you should apply render (a sand and cement mixture), which is hardwearing and can be treated to increase damp-resistance. If your walls are very damp, such as in a basement, you might consider a cement-based waterproof membrane as a basecoat. Non-porous walls that have been sealed with a PVA solution can have a bonding plaster applied as an undercoat.

To maintain the correct depth of plaster, temporarily fix battens to the wall, 1m apart.
Hold the feather edge firmly against the battens to smooth the plaster and achieve the correct thickness.

Applying the basecoat

  • To help you to plaster to the correct depth, fix battens vertically to the wall. The battens should reach from the floor to the ceiling and be10mm (3/8in) thick.  
  • Fix the battens at regular intervals using a spirit level to make sure they are vertical. Temporarily fix the battens to the wall 1m (3ft 3in) apart with masonry screws.
  • Splash the wall’s surface with water, as a dry wall will draw the water out of the plaster.
  • Follow the manufacturer’s instructions to mix the basecoat plaster. This will involve adding the plaster to a bucket half-filled with water and mixing until you have a smooth but stiff consistency. Plaster can be mixed using a power stirrer or manually using an off-cut of batten.
  • Load some of the plaster onto a hawk. Using a plastering trowel push a section of the plaster towards the edge of the hawk, then tilting the hawk transfer this to the face of the trowel.
  • Working on one section of the wall at a time, spread the plaster from the hawk to the wall as evenly as possible trying to maintain the 10mm (3/8in) depth. Continue until one section of the wall is covered with basecoat plaster.
  • Now smooth the plaster with a feather edge to remove excess plaster and fill in hollows. The ends of the feather edge should be held firmly against the battens to achieve the 10mm (3/8in) thickness.
  • Check your basecoat has a level and smooth surface, applying more plaster to hollows if necessary.
  • Repeat this process for each section of the wall.
  • Allow the plaster basecoat to dry. When it is firm but offering a very slight amount of give when finger pressure is applied, remove the battens and fill in the channels they have left. Use the plastering trowel for this purpose ensuring the channels are filled and smoothed to the 10mm (3/8in) depth.

Scoring the basecoat

  • Before the basecoat dries completely it needs to be scored to create a key for the finish coat. A simple wooden float with nails protruding from its face is ideal for this purpose and is easily made.
  • With the face of the modified wooden float against the wall, use a sweeping motion to move the float over the surface of the basecoat. Once the surface has been scored allow the basecoat to go off without it drying out completely. This should take approximately two hours but it does depend on the room temperature, so it’s advisable to check the plaster at regular intervals.
When applying the basecoat plaster to the wall it must lie level with the point of the angle bead.
Use the edge of the plastering trowel to scrape away about 2mmof the basecoat near the angle bead to allow for the layer of finishing plaster.

Plastering external corners

  • To plaster over any external corners, you need to create a base for the plaster by fixing lengths of right-angled metal corner supports called angle bead to the corners. These will also give a sharp neat finish to the corner.
  • Measure the distance from the floor to the ceiling and cut the angle bead to the required length. 
  • Apply a small amount of basecoat at regular intervals – approximately 1m (3ft) apart – on both surfaces of the adjoining walls in order to secure the angle bead in place over the corner.
  • Push the angle bead into the plaster ensuring it is held fast and vertical (check with a spirit level).
  • When applying the basecoat plaster to the wall it must lie level with the point of the angle bead. After the basecoat has gone off, use the edge of the plastering trowel to scrape away about 2mm (1/10in) of the basecoat near to the angle bead to allow for the layer of finishing plaster.












Finishing coats

There are a variety of decorative plasters available that can be used as a top coat but the most common is Finishing Plaster. Whichever type of plaster you are using the application technique is the same.

Hold the hawk close to the wall and use a plastering trowel to transfer the plaster from the hawk to the top of the wall.
Let the face of the plastering trowel rest on the apex of the angle bead at external corners to maintain a professional finish.

Applying the finishing coat

Applying a finishing coat is known as skimming and should be carried out when the basecoat is still slightly damp. If the basecoat is drying out too fast, moisten it by splashing water over its surface.

  • To mix the finishing plaster, half-fill a clean bucket with water and slowly pour in the plaster. Follow the manufacturer’s mixing instructions.
  • As with the basecoat you can mix manually or with a power stirrer, but this must be operated at slow speed and only turned on when it is submerged.
  • Continue adding the plaster to the water and mixing until you have a creamy consistency. Any dry plaster sticking to the inside of the bucket must be loosened by dragging the edge of a pointing trowel around the inside of the bucket and mixing it in.
  • Pour out the finishing plaster onto a large clean board called a spot board. If mixed to the correct consistency it will spread over the board without flowing over the edges.
  • Load your hawk with plaster and, with the hawk held close to the wall, use a plastering trowel to transfer the plaster from the hawk to the top of the wall. As you work you will gradually build up a rhythm.
  • Spread plaster over the wall from top to bottom, using smooth strokes to spread the plaster evenly. Work as quickly as possible to cover the whole wall before the plaster begins to dry. 
  • When the whole wall is covered, smooth over the surface to achieve an even thickness. Allow the plaster to dry for half an hour.
  • Smooth over the plaster again with a clean, damp trowel, filling any small indentations. To achieve a smooth finish the trowel should be held at a slight angle, so that only one edge is touching the plaster.
  • Let the plaster dry out for another half hour before smoothing over the finish surface for a final time. If necessary splash water onto the wall with a plasterer’s brush to keep the plaster damp. If the walls still have an uneven finish, apply a thin layer of plaster on top, and smooth it as before.
  • When applying finishing plaster to external corners, let the face of the plastering trowel rest on the apex of the angle bead as you smooth the plaster to get a professional finish. 
  • Run a small damp paintbrush along the edges where the wall meets the ceiling and floor, and down the line of internal corners to produce a smooth finish.

Tools needed

  • Spirit level
  • Screwdriver
  • Power stirrer
  • Spot board
  • Plasterer’s brush
  • Plastering trowel
  • Hawk
  • Feather edger
  • Wooden float with nails protruding from its face
  • Pointing trowel

Materials needed

  • Timber battens
  • Masonry screws
  • Render or base coat plaster
  • Angle bead
  • Skimming plaster

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